The present invention relates to a copper-based metal alloy containing zinc and aluminium in quantities such as to form a brass characterised, after suitable high temperature homogenisation treatment and tempering, by a crystalline structure of Beta type; in particular the invention relates to an alloy of the said type also containing further alloying elements the functionof which is to reduce the grain size of the alloy itself.
It is known that alloys of the Cu-Zn-Al system of appropriate composition, after suitable solution heat treatment and tempering, manifest a Beta-type structure referred to as "aluminium-Beta-brasses". These brasses are particularly interesting for some of their particular physical and mechanical characteristics such as a high capacity for damping, pseudo-elastic or super-elastic effect, and the shape memory effect both the irreversible or "one way" effect and the reversible or "two way" effect. This latter characteristic particularly qualifies such alloys for the full title of SME, namely the "shape memory effect" or form memory.
As is likewise known, such properties, and in particular the shape memory effect, are allied to a martensitic transition phase of thermo-elastic type, or rather to the formation and growth within the "Beta" structure of martensitic plates; this phase transformation is reversible and controlled by the temperature and elastic stress state of the material. In the absence of mechanical stresses it is characterised by two pairs of initial and final transformation temperatures, respectively indicated Ms and Mf (of the martensitic Beta phase) and As and Af (in the reverse transformation). The interest shown in the above-mentioned effects manifested by "Beta" brasses, and in particular those connected with the shape memory effect and the super-elastic effect, is essentially bound up with the fact that the materials in question are able to perform simultaneously the functions of heat sensor and mechanical actuator. In other words an SME element performs the functions conventionally fulfilled by a complex chain of devices (for example heat sensor, amplifier, relay/proportional actuator, etc).
In such applications the materials in question are subjected to thermo-mechanical stresses of cyclic type and can consequently manifest fatigue phenomena of thermo-mechanical type if suitable arrangements are not adopted. It is known that an essential condition for achieving a good behavior of metal materials when faced with fatigue in general and thermo-mechanical fatigue in particular, is obtaining a very fine and homogeneous grain structure.
Beta-brasses which do not have grain size reducing addition elements have, on the other hand, a decidedly large grain structure and are therefore of low reliability in the long term in thermo-mechanical fatigue conditions.